Hang around the Frontier area and you should see lots of convoys spawning.

They are only guarded by a couple of soldiers and a drummer and you can usually find some loot on the carriage (which is towed by a horse).

Once good spot to get a spawned convoy is in front of the Templar Fort in the south-west of the map (in the very corner).

If it's not there, then just fast travel somewhere else (there is a station at the harbor nearby) and then go back.

You need to interact at the back of the carriage to grab the goodies!